1. Field of the Invention
This invention generally relates to a hollow pipe or a solid shaft made of a fiber-reinforced plastic material (hereinafter referred to as a "bar-like molding make of a fiber-reinforced plastic material") and a method of manufacturing the same. More specifically, this invention relates to a bar-like molding made of a fiber-reinforced plastic material which can be extended through connection, and in particular to one which is suitable for transmitting torque and impulsive force. For example, it is suited to be used as a component of the connected drilling pipe assembly of a boring machine in the fields of civel engineering, oil drilling, etc., or as a propeller shaft for transmitting the torque of a transport machine such as an automobile, a ship or an airplane. It goes without saying that the use of the bar-like molding made of a fiber-reinforced plastic material of this invention is not limited to the transmission of torque or impulsion. It is also suited for use as a high-strength structural material subjected to tensile force and compressive force such as pillars and beams in a building or piping the transporting fluids.
2. Description of the Prior Art
As the connected drilling pipes of a boring machine, for example, steel pipes have conventionally been employed. When using steel pipes as the components of a connected pipe assembly, i.e., as drilling pipes, a male screw is formed at one end of each pipe and a female screw at the other end thereof previously, adjacent pipes being directly connected to each other by engaging the male and female screws formed at the ends. By thus connecting pipes, a pipe assembly of any desired length can be obtained. Alternatively, both ends of each pipe may be formed as female screws, adjacent screws being connected to each other through the intermediary of a connecting short pipe having male screws.
Being made of steel, conventional drilling pipes are attended by the following problems:
(1) Since they are relatively heavy, the operation of connecting them together for extension is labor intensive. This is especially true if the working site is located in a tunnel or on a slope of a mountain. Under such bad conditions, various problems such as prolonged working time and danger in conveying and connecting the pipes may be involved. Their heaviness is inconvenient not only in terms of operational efficiency; if a boring operation is to be performed as deep as several thousands of meters, the weight of the drilling pipes themselves will amount to several hundreds of tons, so that a special support structure may be required for them.
(2) When acid treatment is performed, as in boring for petroleum and natural gas, the hydrochloric acid used will corrode the drilling pipes.
With a view to overcoming these problems, the applicant of the present invention proposed an extension pipe made of fiber-reinforced plastic materials which comprises a hollow pipe made of fiber-reinforced plastic materials and connecting short length metal pipes having a predetermined length and attached to the ends of the hollow pipe, the short length metal pipes including male or female screws for connection (see Japanese Patent Application Kokai Nos. 63-167185 and 63-167184).
The fiber-reinforced plastic pipe proposed is quite advantageous in that it can well withstand percussions and that its weight can be about 1/2 of a conventional steel pipe. However, it is not satisfactory as a pipe for transmitting a high torque.
In this regard, the applicant of this invention discovered the fact that a considerably high torque amounting up to 600 kg.m can be transmitted when an uneven surface configuration is imparted by knurlizing or the like to the outer peripheral surface of a connecting short length metal pipe adapted to be engaged with the inner peripheral surfaces at the ends of a hollow fiber-reinforced plastic pipe, and proposed a fiber-reinforced plastic pipe accordingly constructed (Japanese Patent Application No. 62-260754).
It has been found, however, that, when used for percussion boring, the fiber-reinforced plastic material pipe disclosed in Japanese Patent Application Kokai Nos. 63-167185 and 63-167184 and that proposed in Japanese Patent Application No. 62-206574 can provide only a lower boring speed, i.e., a poorer boring efficiency, as compared with conventional steel boring casings.
The inventors of the present invention conducted a number of experiments with a view to improving the boring efficiency of a fiber-reinforced plastic pipe, and have discovered the fact that, by increasing its axial stiffness, the impulsion transmitting properties of a fiber-reinforced plastic pipe can be augmented, thereby remarkably improving the boring efficiency thereof. In some cases, a boring casing utilizing fiber-reinforced plastic pipes can provide a boring efficiency equal or superior to that provided by conventional boring casings using steels.
The present invention is based on this newly obtained knowledge.